Rotary screw compressor selection guide

Why horsepower is the wrong starting point

Many compressors in Indian plants are chosen the same way: a kW or HP figure from the old machine, a price comparison, and a purchase. The result is often a compressor that is oversized and runs unloaded for half the shift, or undersized and set at a higher pressure to compensate. Both cost energy every hour the plant runs.

The right compressor is not automatically the one with the highest cfm, the highest pressure, the newest control or the lowest price. It is the one that delivers the required free air delivery (FAD) at the required pressure, for the actual duty and demand profile, at the required air quality, inside a correctly engineered system, at an acceptable lifecycle cost. Energy is normally the largest part of that lifecycle cost, so a selection error repeats itself on every electricity bill.

The method below follows that order: application, flow, pressure, duty, air quality, demand profile, site conditions, reliability, system architecture, verified equipment, lifecycle. For the basics of the machine itself, see how rotary screw compression works and our rotary screw compressors hub.

Step 1: list what the air will operate

Build an air-user schedule before you look at any compressor. For each user, record:

• the equipment (CNC machine, packaging line, cylinders, assembly tools, process blowing, instruments, cleaning, conveying);
• its air consumption at rated pressure, from the equipment documentation or measurement;
• the minimum pressure it needs at its inlet;
• how often it runs and how many units run together (simultaneity);
• the air quality it needs.

Do not estimate where verified data can be obtained. Machine builders publish consumption for most pneumatic equipment, and an existing plant can be measured. A short logging survey of the current system, recording flow, pressure and the compressors’ load and unload times, usually tells you more than any spreadsheet. Our specific-power assessment describes how we measure input power against delivered air.

Step 2: calculate flow, including leaks and growth

Add the simultaneous demand of all users, then add three allowances separately so each can be challenged:

1. Leakage. Existing plants always leak. Measure it, for example by running the compressors with production stopped and timing load and unload cycles, rather than assuming a figure. Fix leaks before you buy capacity to feed them.
2. Future growth. Only planned, approved expansion. Speculative margin should be handled by space and pipework for a future machine, not by oversizing today’s.
3. Site derating. FAD is rated at stated reference conditions. High intake temperature, dust and altitude reduce real output and margin.

Always compare compressors by FAD measured to a recognised test basis (such as ISO 1217), at your working pressure. Displacement figures, motor kW and catalogue “capacity” without a pressure are not comparable.

Step 3: fix the lowest pressure that works

Start from the highest pressure any important user needs at its inlet, then add the pressure losses through dryer, filters, piping, hoses and regulators to arrive at the compressor discharge pressure. Every extra bar costs roughly 7% more power for the same air, so a plant running at 8.5 bar when 7 bar would do is paying for it continuously.

• If one user needs much higher pressure than the rest, consider a local booster or a separate machine rather than raising the whole plant.
• If pressure drops at the far end of the plant, fix pipe size and filter condition before raising the setpoint.
• Low-pressure blowing or conveying users (below about 4–5 bar) may justify a low-pressure rotary screw compressor instead of throttling 7 bar air.

See working pressure and pressure budget and the pressure budget builder. For pressure bands, compare medium-pressure and high-pressure screw compressors.

Step 4: understand duty and demand profile

Two plants with the same average demand can need very different compressors. A plant that runs three shifts at a steady load suits a fixed-speed machine running fully loaded. A plant with heavy day-shift demand, a light night shift and frequent swings suits a variable-speed drive or a combination of machines.

A VSD is not automatically the better buy. On a constant full-load duty it adds drive losses and cost for no saving. Choose it from the measured demand profile.

Step 5: define air quality and treatment

Set the required class to ISO 8573-1 for particles, water and oil, by end use, rather than treating the whole plant to the strictest standard. A typical refrigerated dryer gives a pressure dew point of about +3 °C, adequate for most general plant air in Indian conditions. Desiccant dryers give about −20 °C or −40 °C for instrument air, outdoor lines in cold areas, or sensitive processes. Where oil cannot be tolerated at all, compare oil-free rotary screw compressors against oil-injected machines with appropriate filtration; see oil-injected rotary screw compressors. For smaller plants, an integrated compressor and dryer package saves space and installation work.

Step 6: check site conditions

• Ambient temperature. Compressor rooms in Hyderabad and across the Deccan can reach 40–45 °C in summer. Check the package’s maximum ambient rating and plan ventilation so hot cooling air is ducted out, not recirculated. Normal oil-injected screw discharge temperature is about 75–95 °C; poor ventilation pushes it towards the high-temperature trip.
• Dust. Cement, foundry, textile and mineral plants need heavy-duty intake filtration and more frequent cooler cleaning.
• Cooling method. Air-cooled is the norm; water-cooled suits larger machines where good cooling water is available.
• Electrical supply. Voltage stability, starter type and harmonics for VSD machines; DG backup if the plant must run during outages.
• Heat recovery. Most of the input energy ends up as heat. If the plant needs hot water, evaluate heat-recovery systems at selection, not afterwards.

Step 7: choose machine architecture and drive

With flow, pressure, profile, quality and site defined, you can now pick the hardware:

• Size class. Small, medium or large industrial machines, one large unit or several smaller ones.
• Compression stages. Single-stage for most duties; two-stage can improve specific power on larger, high-hour machines, at higher purchase cost.
• Drive. Belt, gear or direct drive, and permanent-magnet motors on some VSD designs.
• Layout. Floor-standing or tank-mounted for small installations; a correctly sized receiver in all cases.

Compare candidates on specific power (kW input per m³/min of FAD) at your pressure and at your expected part load, not just at full load. See specific power.

Worked example: a medium engineering plant

The figures below are illustrative, to show the method; your own survey data replaces them.

A two-shift engineering plant lists its users: CNC machines and tool changers, assembly tools, a paint booth and general blowing. The air-user schedule gives a simultaneous demand of about 8 m³/min (about 280 cfm) on the day shift and about 4 m³/min on the evening shift. A leak test with production stopped shows the system using about 1.5 m³/min, and an approved new line will add about 1 m³/min next year.

1. Flow. Fix the worst leaks first, aiming to halve them. Day-shift requirement: 8 + 0.75 (remaining leaks) + 1 (growth) = about 9.75 m³/min, roughly 345 cfm.
2. Pressure. The most demanding user needs 6 bar at its inlet. Losses through dryer, filters and piping are measured at about 0.8 bar. Discharge setpoint: about 7 bar, not the 8.5 bar the old machine ran at. That alone reduces power by roughly 10% for the same air.
3. Profile. Demand swings between about 4 and 10 m³/min. A single fixed-speed machine sized for the peak would run unloaded for much of the evening shift.
4. Architecture. One fixed-speed machine of about 6 m³/min as base load plus a VSD machine of about 5–6 m³/min as trim. Together they cover the peak; either one can keep essential users running during service.
5. Air quality. Refrigerated dryer at about +3 °C PDP for general air; extra coalescing filtration at the paint booth only.
6. Site. Air-cooled machines, with a ducted hot-air exhaust from the compressor room because summer room temperature reaches the mid-40s °C.
7. Verification. Ask each supplier for FAD and input power at 7 bar to a recognised test basis, and for VSD part-load data, then compare annual energy on the actual profile.

The plant ends up with less installed capacity than a “replace like for like” purchase, at a lower pressure, with redundancy it did not have before.

Common selection mistakes

• Selecting by motor HP or by copying the old machine’s nameplate.
• Comparing catalogue capacity at different pressures or without a stated test basis.
• Raising pressure to hide undersized piping or blocked filters.
• Buying capacity to feed leaks instead of fixing them.
• Choosing a VSD for a constant full-load duty, or a fixed-speed machine for a highly variable one.
• Treating all plant air to instrument-air quality, or ignoring oil and water where the process cannot accept them.
• Ignoring compressor-room ventilation in an Indian summer.
• Single large machine with no backup for a critical process.
• Comparing purchase price only, when energy and maintenance dominate lifecycle cost.

Selection checklist

• Air-user schedule with consumption, pressure, simultaneity and quality.
• Measured demand profile and leak rate for an existing plant.
• Pressure budget from the compressor to the most demanding user.
• ISO 8573-1 quality class for each air zone; dryer and filter plan.
• Site data: ambient, dust, cooling, electrical supply, space, heat-recovery use.
• Architecture: number of machines, base and trim, redundancy, controls.
• FAD and specific power at your pressure and part load, to a recognised test basis.
• Service plan, spares availability and lifecycle cost estimate.

After installation, the plan continues with rotary screw service and lifecycle care and, where suitable, an AMC.

How Reddy Compressors helps

Reddy Compressors, Tarnaka, Secunderabad, has served industry since 2002. As an independent multi-brand specialist, not an authorised dealer, we sell, rent and service rotary screw compressors from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and others, so a selection can be compared across makes on your duty. We have tie-ups with all major and reputed brands.

• Demand and pressure surveys, leak checks and specific-power assessment on existing systems.
• Selection advice and supply of new compressors, dryers and filters; used machines, test-run under load, where they fit.
• Rental compressors to cover the plant during changeover or while a decision is made.
• Installation guidance, commissioning, service, AMC and spares, with more than 20 service engineers and technicians covering Telangana and Andhra Pradesh; equipment and spares supplied across India.

What to send us

• List of air users with consumption and pressure, or the plant’s production equipment list.
• Existing compressors: make, model, nameplate, running hours, pressure setting.
• Shift pattern and how demand changes through the day.
• Air-quality needs (general, painting, instrument, food or pharma contact).
• Compressor-room photos, ambient conditions and electrical supply details.
• Planned expansion and whether backup is needed.

Frequently asked questions

How do I calculate what size rotary screw compressor I need?

Add the air consumption of all users that run at the same time, at their rated pressure, then add measured leakage, approved growth and an allowance for site conditions. Express the result as free air delivery in m³/min or cfm at your working pressure. For an existing plant, a logging survey of flow, pressure and compressor load cycles gives the most reliable figure.

Should I buy a VSD or a fixed-speed screw compressor?

Choose a VSD when demand varies significantly through the day, because motor speed follows demand and avoids power wasted running unloaded. Choose fixed speed when the machine will run close to full load for long hours, since a VSD then adds cost and drive losses without much saving. Many plants combine fixed-speed base machines with one VSD trim unit.

What pressure should my screw compressor be set at?

Set it at the lowest pressure that still delivers what the most demanding important user needs at its inlet, after allowing for losses through dryers, filters and piping. Every additional bar costs roughly 7% more power. If one user needs much higher pressure, consider a booster or separate machine instead of raising the whole system pressure.

Is one large compressor better than two smaller ones?

One large machine can have slightly better full-load efficiency and lower purchase cost, but it leaves the plant without air during breakdown or service. Two or more smaller machines, often with a VSD trim unit, follow variable demand better and provide redundancy. The right answer depends on your demand profile and how critical continuous air is.

Do I need an oil-free screw compressor?

You need oil-free compression where any oil in the air is unacceptable, such as some food, pharma, electronics and process applications, or where a customer or standard requires it. Many general industrial uses are served well by an oil-injected machine with suitable filtration to the required ISO 8573-1 class. Decide by end use and risk, not by habit.

What is specific power and why does it matter when comparing compressors?

Specific power is the electrical input in kW divided by the free air delivered in m³/min, at a stated pressure. Lower is better. It lets you compare machines of different makes and sizes on energy use, which usually dominates lifecycle cost. Compare it at your actual working pressure and expected part load, to a recognised test basis.

Related pages

• Rotary screw compressors hub
• Variable-speed drive rotary screw compressors
• Rotary screw compressor controls and sequencing
• Rotary screw vs reciprocating air compressors
• Specific power and part-load performance
• Rotary screw service and lifecycle